Picture signal processing circuit for improving high-frequency picture resolution upon picture signal display

ABSTRACT

The circuit has an input terminal (1) for supplying a picture signal (CS) formed line and field-sequentially with line and field or frame periods (TV) and an output terminal (2) for supplying a processed picture signal (CS&#39;) to be applied to a picture display device (5) comprising a picture display tube operating by way of electron beam scanning (e) of a display screen (8). In the circuit the input terminal (1) is coupled to a filter circuit (10) which has a first output (11) and a second output (12) for supplying a low-frequency (CS L ) or high-frequency (CS H ) picture signal component, respectively. The first output (11) is coupled via an amplifier circuit (16) to a first input (21) of a time-division multiplex circuit (20) having a second input (22) to which the first output (11) and the second output (12) of the filter circuit (10) are coupled each via an amplifier circuit (17, 18) and an adder circuit (19). The time-division multiplex circuit (20) has a switching input (23) which is coupled to an output (29) of a switching signal generator (28) for supplying a periodical switching signal (VS) of the double field or frame period (2TV). An output (24) of the time-division multiplex circuit (20) is coupled to the output terminal (2) of the picture signal processing circuit. The resultant spatial time-division multiplex display yields the improved high-frequency picture resolution.

BACKGROUND OF THE INVENTION

The invention relates to a picture signal processing circuit forimproving high-frequency picture resolution upon picture signal display,which circuit has an input terminal for supplying a picture signalformed line and field-sequentially with line and field or frame periodsand an output terminal for supplying a processed picture signal to beapplied to a picture display device comprising a picture display tubeoperating by way of electron beam scanning of a picture display screen.

Such a circuit for improving the high-frequency picture resolution isdesired due to the use of electron beam scanning upon picture signaldisplay. With electron beam scanning larger picture brightnesses andstronger contrasts are accompanied by an enlargement of the electronbeam spot on the display screen. This leads to a deterioration of thehigh-frequency picture resolution.

SUMMARY OF THE INVENTION

The invention has for its object to realize a picture signal processingcircuit leading to an optimum high-frequency picture resolution. To thisend a circuit according to the invention is characterized in that theinput terminal in the circuit is coupled to a filter circuit for thepicture signal, which filter circuit has a first and a second output forsupplying a low-frequency and a high-frequency picture signal component,respectively, the first output being coupled via an amplifier circuit toa first input of a time-division multiplex circuit having a second inputto which the first and the second output of the filter circuit arecoupled each via an amplifier circuit and an adder circuit, saidtime-division multiplex circuit having a switching input which iscoupled to an output of a switching signal generator for supplying aperiodical switching signal of the double field or frame period, anoutput of the time-division multiplex circuit being coupled to theoutput terminal of the picture signal processing circuit.

It is achieved by the circuit that low-frequency and high-frequencypicture signal components are displayed in an alternating manner,separated in time, which can be designated as "spatial time-divisionmultiplex". In this case a presupposition is made for the circuit,namely: no maximum modulation depth is present in the high-frequencypicture signal component. In practice this presupposition holds true forsubstantially all picture signal sources, except for electronic testsignal generators which can supply high-frequency picture signals at amodulation depth of 100%.

A circuit according to the invention, which operates in an optimummanner also if such a test picture signal is possibly supplied, ischaracterized in that the circuit comprises a circuit for measuringmodulation depth, an input of said measuring circuit being coupled tothe second output of the filter circuit for supplying the high-frequencypicture signal component and an output being coupled to an input of thesaid generator, the time-division multiplex circuit being renderedinoperative when a maximum modulation depth is measured and ashort-circuit switch arranged between input terminal and output terminalbeing rendered operative. The short-circuit between input terminal andoutput terminal leads to the then optimum possible picture resolution.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in greater detail by way of example withreference to the accompanying drawing whose sole FIGURE shows a picturesignal processing circuit according to the invention in ablock-schematic diagram.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

In the FIGURE the reference numeral 1 denotes an input terminal and 2denotes an output terminal of the picture signal processing circuit. Thereference numeral 3 denotes an input terminal for supplying a videosignal CVHS to be processed. The signal CVHS comprises a line andfield-sequentially composed picture signal CS and a synchronizing signalVHS which comprises, for example line and field-synchronizing pulses,blanking pulses and equalizing pulses. The video signal CVHS may befixed or not fixed in a television standard and upon display it maypresent a non-interlaced picture or a single or multiple interlacedpicture. In the case of single interlacing there is a field period of,for example 20 s or 16.6833 s and a line period of 64 μs or 63.555 μs,respectively, the frame period being equal to the double field period.In the case of non-interlacing the frame period is identical to thefield period and in the case of 20 ms a frame or field frequency of 50Hz is associated therewith. To reduce flickering phenomena upon picturedisplay, the field frequency or field period may be chosen to be equalto 100 Hz or 10 ms, respectively, in the case of interlacing ornon-interlacing. The specific composition of the video signal CVHS isfurther disregarded. The line and field-sequentially composed picturesignal CS is assumed to be originating from a picture signal sourceforming part of a television system in its widest sense. Examples arebraodcast television, X-ray or infrared television, television formonitoring purposes, for displaying computer data etc. In this casethere may be a display with moving or stationary pictures. Furthermorethe picture signal CS may be a luminance, chrominance or colourdifference signal in the case of colour television or a picture signalin the case of monochrome television.

In the Figure the reference numeral 4 denotes a signal separatingcircuit which operates inter alia as a signal separating circuit forsupplying the picture signal CS and the field and line-synchronizingsignal VHS when the video signal CVHS is supplied. Furthermore thesignal separating circuit 4 may comprise signal correction circuits suchas a de-gamma correction circuit. If the signal CVHS is a digitalsignal, the signal separating circuit 4 may comprise a D/A converterwhen the signal processing circuit with the input terminal 1 and theoutput terminal 2 is operative on an analog basis. If this circuitoperates on a digital basis and if an analog signal CVHS is presented,the signal separating circuit 4 will comprise an A/D converter.

The Figure shows a picture display device denoted by the referencenumeral 5, an input 6 of which is coupled to the signal separatingcircuit 4 for supplying the signal VHS. An input 7 is coupled to theoutput terminal 2 for supplying the processed picture signal which isdenoted by CS'. The device 5 is shown with a display screen 8 in whichtwo picture signals CS1 and CS2 are shown diagrammatically by way ofexample. The signal CS1 is shown with a small amplitude and it isdisplayed by means of an electron beam with a small beam spot e1 on thescreen 8. The signal CS2 is shown with a larger amplitude and isassociated with a larger electron beam spot e2. The electron beam e isgenerated by an electron gun (not shown) which is present in an electrondisplay tube which forms part of the display device 5. The displayscreen 8 of the display tube is scanned by the electron beam e under thecontrol of line and field deflection means such as, for exampledeflection coils. The diameter of the spot e1 or e2 is dependent on theinstantaneous picture signal amplitude. In this case a larger amplitudeis accompanied by a reduced picture resolution which is fixed by thespot size. The weaker picture signal CS1 has a higher picture resolutionupon display than the stronger picture signal CS2. According to theinvention the stronger picture signal CS2 will be displayed with animproved high-frequency picture resolution due to the formation of thepicture signal CS'.

The input terminal 1 is coupled to an input 9 of a filter circuit 10which has a first output 11 and a second output 12. The input 9 iscoupled via a capacitor 13 to the output 12 which is further coupled toground via a resistor 14. The capacitor 13 and the resistor 14 combinedconstitute a high-pass filter (13, 14) in its simplest form. The filtercircuit 10 also comprises a subtractor circuit 15 having a (+) inputwhich is coupled to the input 9 and a (-) input which is coupled to theoutput 12. An output of the circuit 15 is coupled to the filter output11. In conjunction with the filter (13, 14) the circuit 15 operates as alow-pass filter (13, 14, 15). The embodiment shown means that the filtercircuit 10 comprises complementary filters (13, 14) and (13, 14, 15).The result is that the picture signal CS is split into a low-frequencypicture signal component CS_(L) and a high-frequency picture signalcomponent CS_(H), which are complementary. Instead of the embodimentshown, two separate low-pass and high-pass filters could be used whichare complementary or not complementary. Overlapping filtercharacteristics or separated, non-contiguous filter characteristics maybe present in this case. In practice complementary filtercharacteristics for the high-frequency resolution improvement appear tobe preferred.

The output 11 is coupled to an input of a first amplifier circuit 16 anda second amplifier circuit 17. The output 12 is coupled to an input of athird amplifier circuit 18. Outputs of the circuits 17 and 18 arecoupled to (+) inputs of an adder circuit 19. The circuits 16, 17 and 18are shown with adjustable gain factors denoted p, q and r, respectively.The factors p, q and r may be adjustable or dynamically adjustable,dependent on the picture signal contents. Fixed factors p, q and rdetermined by calculation could also be present. The adjustability is apossible option.

The reference numeral 20 in the Figure denotes a time-division multiplexcircuit. A first input 21 and a second input 22 are coupled to an outputof the circuits 16 and 19, respectively, which supply a signal p.CS_(L)and q.CS_(L) +r.CS_(H), respectively. The circuit 20 has a switchinginput 23 for applying a switching signal VS and an output 24 forsupplying the processed picture signal CS'. The circuit 20 is shown witha first switch 25 and a second switch 26, the switching signal VScontrolling the switch 25 directly and the switch 26 via an invertercircuit 27. Consequently, in the open or closed state of the switch 25,the switch 26 will be closed or open, respectively. For the sake ofsimplicity the switches 25 and 26 are shown as mechanical switches, butin practice they will be electronic switches. It appears that theprocessed picture signal CS' which is supplied from the output terminal2 via the output 24 of the circuit 20 has undergone a time-divisionmultiplex signal processing. To this end the signal VS to be supplied bya switching signal generator 28 is applied to the switching input 23. Afirst output 29 of the generator 28 supplies the switching signal VS,while a short-circuit signal S is available at a second output 30. Thesynchronizing signal VHS is applied to a first input 31 of the generator28. The result is the supply of the switching signal VS shown. Two fieldor frame periods TV are denoted by TV1 and TV2 at the signal VS. Thesignal VS shows a periodical switching signal of the double field orframe period 2TV. The short-circuit signal S is supplied by thegenerator 28 when the switching signal VS is absent, more specificallywhile applying a signal to a second input 32 of the generator 28, whichsignal originates from a circuit 33 for measuring modulation depth.

The circuit 33 for measuring modulation depth has an input 34 which iscoupled to the output 12 of the filter circuit 10 and to an output 35 tobe coupled to the input 32 of the generator 28. In the circuit 33 themeasurement of the modulation depth is illustrated by M and O. In caseof a measurement of a high modulation depth in the high-frequencypicture signal component CS_(H) the generator 28 supplies theshort-circuit signal S to be applied to a short-circuit switch 36. Theswitch 36 has an input 37 which is connected to the input terminal 1,while a short-circuit input 38 is coupled to the output 30. The switch36 is shown with a switching member 39 which is controlled via theshort-circuit input 38. An output 40 is connected to the output terminal2. When measuring a given maximum modulation depth in the high-frequencypicture signal component, the time-division multiplex circuit 20 isrendered inoperative, while the short-circuit switch 36 is renderedoperative and the switching member 39 connects the input terminal 1 tothe output terminal 2.

The block-schematic diagram of the Figure shows that, for example thepicture signal component p.CS_(L) during first periods TV1 and then thepicture signal component q.CS_(L) +r.CS_(H) during second periods TV2are periodically applied for display to the device 5 with a period 2TVwhich is equal to the double field or frame period TV. For the picturesignal CS2 shown this means that a picture signal component CS2₁ whichis present under a broken line is displayed during the first periodsTV1, while a component CS2₂ which is present above the broken line isdisplayed during the second periods TV2. In this case it holds that CS2₁=p.CS_(L) and CS2₂ =q.CS_(L) +r.CS_(H). The component CS2₁ is displayedwith the electron beam spot e2, while the component CS2₂ is displayedwith the spot e1. Consequently, the low-frequency picture information isdisplayed with the low picture resolution, while the combined high andlow-frequency picture information is displayed with a higher pictureresolution. The "spatial time-division multiplex display" in whichpicture signal components are displayed in an alternating manner,separated in time, leads to an improved high-frequency pictureresolution.

The picture resolution improvement is based on the presupposition thatthe high-frequency picture signal component CS_(H) does not have amaximum modulation depth of 100%. In practice, this presupposition holdstrue for substantially all picture signal sources, except for electronictest signal generators as a picture signal source. If the picture signalcomponent CS_(H) has a high modulation depth and if this component isdisplayed with the larger electron beam spot e2, the picture appears tohave a poorer quality in practice. For this purpose the short-circuitswitch 36 is provided in the circuit according to the invention, becausethen the original picture resolution is the optimum possible pictureresolution.

In the picture signal combination CS2₂ =q.CS_(L) +r.CS_(H) thelow-frequency component CS_(L) is present as a pedestal for thehigh-frequency component CS_(H). This pedestal is required to ensurethat the total signal CS2₂ is always positive.

It appears that the spatial time-division multiplex display is effectedwith a period which is equal to the double field period TV. Startingfrom a period TV=10 ms, the signal CS2₁ =p.CS_(L) is displayed during afirst 10 ms and the signal CS2₂ =q.CS_(L) =r.CS_(H) is displayed duringthe second 10 ms. To realise more or less the same brightness in thecase of spatial time-division multiplex display as compared with theknown display, it must hold that p+q and r are approximately equal totwo. In practice the following values of the gain factors appear to besatisfactory: p=1.2, q=0.8 and r=2; p=1.3, q=0.7 and r=2; p=1.2, q=0.8and r=2.5. Other combinations are possible. In the last-mentionedcombination with r=2.5 there is a further accentuation at the display ofthe higher frequencies in the picture signal.

In the case of colour television the picture signal CS is, for example aluminance signal, while associated chrominance or colour differencesignals are directly applied in an unprocessed form to the displaydevice 5.

In the case of projection television the display device 5 may comprise aprojection display tube in which the picture on the screen 8 isprojected on a projection screen via a system of lenses. In this casethe two picture signal components could each be applied to a separateprojection display tube, while a picture combination is formed on theprojection screen via the system of lenses. A high picture brightnessand resolution is the result upon display.

We Claim
 1. A picture signal processing circuit for improvinghigh-frequency picture resolution upon picture signal display, whichcircuit has an input terminal for supplying a picture signal formed lineand field-sequentially with line and field or frame periods and anoutput terminal for supplying a processed picture signal to be appliedto a picture display device comprising a picture display tube operatingby way of electron beam scanning of a picture display screen,characterized in that the input terminal in the circuit is coupled to afilter circuit for the picture signal, which filter circuit has firstand second outputs for supplying a low-frequency and a high-frequencypicture signal components, respectively, the first output being coupledto a first input of a time-division multiplex circuit having a secondinput to which the first and the second outputs of the filter circuitare coupled, said time-division multiplex circuit having a switchinginput which is coupled to an output of a switching signal generator forsupplying a periodical switching signal of the double field or frameperiod, an output of the time-division multiplex circuit being coupledto the output terminal of the picture signal processing circuit.
 2. Apicture signal processing circuit as claimed in claim 1, wherein the lowand high-frequency picture signal components are complementary.
 3. Apicture signal processing circuit as claimed in claim 2 furthercomprising a circuit for measuring modulation depth, an input of saidmeasuring circuit being coupled to the second output of the filtercircuit for supplying the high-frequency picture signal component and anoutput being coupled to an input of said switching signal generator, thetime-division multiplex circuit being rendered inoperative when amaximum modulation depth is measured and a short-circuit switch arrangedbetween input terminal and output terminal being rendered operative. 4.A picture signal processing circuit as claimed in claim 1, furthercomprising a circuit for measuring modulation depth, an input of saidmeasuring circuit being coupled to the second output of the filtercircuit for supplying the high-frequency picture signal component and anoutput being coupled to an input of said switching signal generator, thetime-division multiplex circuit being rendered inoperative when amaximum modulation depth is measured and a short-circuit switch arrangedbetween input terminal and output terminal being rendered operative. 5.A picture signal processing circuit as claimed in claim 1 wherein afirst amplifier circuit is disposed between said first output of saidfilter circuit and said first input of said time-division multiplexcircuit, a second amplifier circuit is disposed between said firstoutput of said filter circuit and a second input of said time-divisionmultiplex circuit, and a third amplifier circuit is disposed betweensaid second output of said filter circuit and said second input of saidtime-division multiplex circuit.
 6. A picture signal processing circuitas claimed in claim 5 wherein the first, second and third amplifiercircuits have adjustable gain factors.
 7. A picture signal processingcircuit as claimed in claim 6 further comprising a circuit for measuringmodulation depth, an input of said measuring circuit being coupled tothe second output of the filter circuit for supplying the high-frequencypicture signal component and an output being coupled to an input of saidswitching signal generator, the time-division multiplex circuit beingrendered inoperative when a maximum modulation depth is measured and ashort-circuit switch arranged between input terminal and output terminalbeing rendered operative.